The computer industry has made extensive use of magnetic disks for data storage, both rigid (or "hard") and flexible (or "floppy") disks or diskettes. Floppy disks, which are commonly used on personal computers (PC's), are generally made of a very thin circular plastic (mylar) base with a magnetic oxide coating placed inside a protective jacket. The standard floppy disk is 51/4" in diameter, although the 31/2" disk is also becoming very common. The floppy disks are rotated during operation. Data is generally recorded on a plurality of relatively closely spaced concentric tracks. A read/write (R/W) head is selectively moved to selected tracks to read or write data on the tracks.
In magnetic disk drives it is desirable to achieve the highest recording density possible, which is a function of both the number of annular tracks per radial inch of disk surface and the number of bits per inch along each track. The optimum number of tracks on the individual disks is dependent on the precision of the head positioning system and on the precision in the alignment of the various read/write heads in the disk drive.
The track density on the standard 51/4" flexible or floppy disks is typically 48 tracks/inch (TPI) with 40 tracks. The amount of data which may be recorded is dependent upon closer spacing of a larger number of tracks. An 80 track format with 96 TPI is also used. Increasing the number of tracks with closer spacing presents limitations with respect to positioning the read/write head. This is especially true when, as is generally the case, the disks are subject to physical changes which produce dimensional variations as a result of changes in the environment such as temperature or humidity.
The normal procedure for assuring that the read/write heads are in alignment is to place a master alignment disk into the disk drive which has pre-recorded and precisely positioned alignment tracks on the disk surface. Such a pre-recorded alignment disk is commercially available for use in head alignment and is usually referred to as "CE" (Customer Engineer) disk. Through means of special track spacing, or eccentrically written tracks on a master diskette (eccentric relative to the disk drive bore center), or a combination of the two, the track center (of a particular track) is found by zeroing in on the master diskette used as a reference tool. Obviously, errors in recording of the master diskette, degradation due to wear, and environmental factors tend to reduce master diskette accuracy in time even without considering the high level of skill required to ensure desired results in use. By reading the signals from the head, as the head reads the alignment track, and by feeding such signals to a display device, the head can be adjusted until it is in precise tracking engagement with the tracks on the surface of the disk.
With the tendency towards higher track densities in floppy disks, the alignment of the read/write head becomes acute and small dimensional changes in the disk could result in the read/write head actually being positioned over the wrong track. The accuracy requirements for "on track" are such that a very minor positional error can result in the inability to interchange diskettes between drives, and accumulative drift can result in loss of information since tracks written on a unit and stored for a period of time, during which a shift due to wear has occurred, can result in the inability to retrieve that data. A failed read/write head or drift due to wear are major problems with pre-exisiting systems not amenable to easy, convenient service on location. Removal of a magnetic head carriage from such units creates a major problem with original alignment destroyed by disassembly.
A failure to properly read a specific piece of data when using a computer system raises several questions. One key question is whether or not the read/write head is in proper alignment. To answer this question, skilled service has been required. To determine disk drive alignment, current practices require removal of the drive from the disk system by a skilled serviceman, who connects the drive to an exerciser, inserts an alignment disk, and with an oscilloscope interprets the pattern to determine if the drive alignment is satisfactory for the purpose intended. This requires several hours of down time for the system and the services of a skilled technician with proper equipment. Head alignment is very important since about one-third of computer failures are caused by the disk drive. Most disk drive problems can be corrected by head alignment, motor speed adjustment or cleaning.
Disks for aligning disk drives with an oscilloscope are well known. A flexible magnetic disk with signals varying on either side of a track centerline is disclosed in U.S. Pat. No. 4,084,201 to Hack, et al. These signals are read and interpreted with the aid of an oscilloscope to check and make track adjustment, ascertain disk eccentricity and spindle eccentricity, check functioning of a magnetic head, and adjust the azimuth angle of the head. U.S. Pat. No. 3,593,331 to Connell, et al., discloses an alignment disk with a three-track arrangement that provides a group of signals which may be read with a plurality of read/write heads and interpreted with the aid of an oscilloscope to produce an electrical function representative of disk alignment.
Automatic control systems for maintaining alignment of transducers are disclosed in U.S. Pat. No. 4,068,267 to Inouye; U.S. Pat. No. 4,149,200 to Card; U.S. Pat. No. 4,157,576 to Hack; U.S. Pat. No. 4,190,859 to Kinjo; and U.S. Pat. No. 4,213,148 to Clemens. U.S. Pat. No. 4,223,189 to Yonezawa, et al., discloses obtaining an alignment signal by analyzing signal variation caused by known undulations in the recording track.
An automated alignment system is the Cateye-Signal Intercept Detect for Aligning a Read/Write Head Above a Computer Data Storage Disk, disclosed in U.S. Pat. No. 4,321,636 to Lenz. An example of a portable field test unit used for performing tests on disk drives that is fairly complex and expensive and requires a skilled technician is disclosed in U.S. Pat. No. 4,348,761 to Berger.
Attempts have been made to write a disk in machine-readable code in an off-track manner. These attempts were based on an assumption that if the drive in question could read a track offset by one-half track width, in either direction from a correct track centerline position, the head alignment must be satisfactory. This assumption is erroneous. A signal degrades by fifty percent (50%) of the signal level for a one-half track width offset condition, and most disk drives will produce error-free signals well beyond this offset because the fifty percent (50%) signal level due to off-track conditions is basic to design in that type of device. Thus, such disks with off-track data will be read with failure points determined only by indeterminate system noise.
When the transducer head of the computer disk drive is "out of alignment" then the disk drive must be repaired. Typically, this repair is carried out by a skilled technician using sophisticated and expensive equipment. Generally, an oscilloscope or other specialized and expensive equipment is required, and it is generally time consuming and expensive to carry out this task of realigning the transducer head relative to the "tracks" on the disk drive. Thus, there has been a continuing need for a small and inexpensive means by which a person with a minimal amount of training can carry out the task of aligning the read/write head of a computer disk drive. With the extremely low cost of currently available floppy disk drives, the need for quick low cost alignment adjustment is particularly acute or it will be cheaper to replace the entire disk drive.
The invention is directed to a simple and inexpensive method and apparatus for radial alignment of the disk drives of computers, in particular the "floppy" disk drives used in personal computers (PC's).
The system of the present invention provides for the precise alignment of the read/write head in the disk drive so that an optimum number of tracks per radial inch of the individual disks may be achieved. This precise alignment can be accomplished by a semi-skilled technician following the relatively simple steps outlined by this invention and using the alignment testing apparatus of this invention.
Thus, this invention solves a long felt need in the computer industry whereby the transducer or read/write heads of disk drives for personal computers can be aligned relatively quickly and easily using the apparatus and method of this invention.